The present invention relates to the transmission of data over telecommunications networks.
The designs of virtually all of the voiceband modems which have been introduced into the marketplace to date have been based on a model of a public switched telephone network channel as being an analog channel from end to end. In such a model, one of the significant noise sources is the quantization noise introduced by so-called PCM vocoders (pulse code modulation voice coders). In particular, in an originating central office, the PCM vocoder converts input analog signals, such as voice signals or analog voiceband data signals (such as QAM signals), into digital form for transmission across digital facilities within the core of the network. At the terminating central office, a matching vocoder reconverts those signals back into analog form. The quantization noise arises from the fact that when the input signal is sampled just prior to quantization, its amplitude is almost never exactly equal to any of the vocoder's predefined quantization levels. Thus what gets transmitted is the quantization level that is closest to the actual signal amplitude--actually an (illustratively) 8-bit word which represents that level. The discrepancy between the actual amplitude and the transmitted representation of that amplitude appears in the receiving modem as the form of noise referred to as quantization noise.
Noise in a channel is an important consideration in the design of a modem--more particularly the design of its signaling and modulation formats--because power constraints imposed by the network on signals applied thereto, combined with the assumed worst-case level of noise give rise, in turn, to a particular worst-case signal-to-noise ratio (SNR) that must be assumed to exist SNR, in turn, is one of the two principal factors which limit the rate at which data can be transmitted over a channel, bandwidth being the other.
Various techniques, including quadrature-amplitude modulation, trellis coding, echo cancellation, and adaptive equalization developed over the last two decades have allowed modem data rates to progress, even in the face of the network's SNR and bandwidth constraints, from about 2.4 kilobits per second (kbps) in the early 1980s to rates in excess of 30 kbps today. Moreover, U.S. Pat. No. 5,406,583 issued Apr. 11, 1995 to N. Dagdeviren, hereby incorporated by reference, teaches that one can completely eliminate the vocoder quantization noise as a source of impairment in the overall channel--and thereby yet further increase the data rates achievable over a public switched telephone network channel--by encoding the data bits to be transmitted using the codes which represent the vocoders' quantization levels and delivering those codes to the network in their digital form. By thus matching the amplitudes of the transmitted signal--actually represented by an 8-bit word--to the predefined quantization levels of the vocoder, the receiving vocoder's analog output amplitude is an exact, rather than an approximated, representation of the input amplitude. In essence, this approach implements a modulated signaling scheme based on a constellation of signal points derived from the quantization levels of the vocoder. Such a constellation is herein referred to as a "PCM-derived constellation."